US7023461B2 - Deconvolution scheme for reducing cross-talk during an in the line printing sequence - Google Patents

Deconvolution scheme for reducing cross-talk during an in the line printing sequence Download PDF

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US7023461B2
US7023461B2 US10/738,816 US73881603A US7023461B2 US 7023461 B2 US7023461 B2 US 7023461B2 US 73881603 A US73881603 A US 73881603A US 7023461 B2 US7023461 B2 US 7023461B2
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pixel
line
heat
nibs
heater elements
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Dirk Verdyck
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Agfa HealthCare NV
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41J—TYPEWRITERS; SELECTIVE PRINTING MECHANISMS, i.e. MECHANISMS PRINTING OTHERWISE THAN FROM A FORME; CORRECTION OF TYPOGRAPHICAL ERRORS
    • B41J2/00—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed
    • B41J2/315—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by selective application of heat to a heat sensitive printing or impression-transfer material
    • B41J2/32—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by selective application of heat to a heat sensitive printing or impression-transfer material using thermal heads
    • B41J2/35—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by selective application of heat to a heat sensitive printing or impression-transfer material using thermal heads providing current or voltage to the thermal head
    • B41J2/355—Control circuits for heating-element selection

Definitions

  • the present invention relates to a method for reducing or eliminating cross-talk when operating a thermal print head for printing one line on a recording medium.
  • the thermal head has energisable heater elements which are individually addressable.
  • the recording medium is a thermographic material, and the head relates to thermal imaging, generally called thermography.
  • Thermal imaging or thermography is a recording process wherein images are generated by the use of imagewise-modulated thermal energy.
  • Thermography is concerned with materials which are not photosensitive, but are sensitive to heat or thermosensitive and wherein imagewise applied heat is sufficient to bring about a visible change in a thermosensitive imaging material, by a chemical or a physical process which changes the optical density.
  • thermographic recording materials are of the chemical type. On heating to a certain conversion temperature, an irreversible chemical reaction takes place and a coloured image is produced.
  • the heating of the thermographic recording material may be originating from image signals which are converted to electric pulses and then through a driver circuit selectively transferred to a thermal print head.
  • the thermal print head consists of microscopic heat resistor elements, which convert the electrical energy into heat via the Joule effect.
  • the electric pulses thus converted into thermal signals manifest themselves as heat transferred to the surface of the thermographic material, e.g. paper, wherein the chemical reaction resulting in colour development takes place.
  • This principle is described in “Handbook of Imaging Materials” (edited by Arthur S. Diamond—Diamond Research Corporation—Ventura, Calif., printed by Marcel Dekker, Inc. 270 Madison Avenue, New York, ed. 1991, p. 498–499).
  • a particular interesting direct thermal imaging element uses an organic silver salt in combination with a reducing agent. An image can be obtained with such a material because under influence of heat the silver salt is developed to metallic silver.
  • a thermal impact printer uses thus heat generated in resistor elements to produce in a certain image forming material, a localised temperature rise at a certain point, which, when driven high enough above a threshold temperature and being kept a certain time above this threshold temperature, gives a visual pixel.
  • a localised temperature rise at a certain point which, when driven high enough above a threshold temperature and being kept a certain time above this threshold temperature, gives a visual pixel.
  • many pixels are being formed in parallel on a same line and then repeated on a line by line basis where the thermographic medium is moved each time over a small position.
  • thermal heads were low resolutions only (120 dpi), but starting from the early 80's, new technological inventions have driven this resolution into the 600 dpi area (e.g. U.S. Pat. No. 4,360,818 or 5,702,188).
  • this technology always puts some constraints on the electrical configuration and the controllability of the individual nibs. This comes from the fact that in most cases the construction is based on a screen printing technology which has limited resolution but gives a low cost and fast manufacturing benefit. Constrained by this limited resolution, special configurations are being used to increase the printing resolution of the thermal head despite some electrical inconveniences:
  • this “time multiplexing” of control electronics in such a head will only lower the printing speed as not all nibs can be excited simultaneously and accordingly, this groups of nibs must be printed one after the other in time.
  • FIG. 1 based on U.S. Pat. No. 5,702,188.
  • every 2 nibs will have a common switch S i to the ground potential, effectively having 1 electronics switch S i for controlling two adjacent nibs.
  • Selection of the left or right nib sharing a same switch S i is done by taking appropriate values of the voltages V a and V b . In this case, a total line can only be printed using two print jobs controlling each time the same electronic switches but having a different set of supply voltages in the two cases.
  • a sub line printing method In each sub line, a specific group or set of heater elements or nibs are being addressed and the combination of all sub lines produces a full graphical line, having addressed all the heater elements over the full printing range of the print head.
  • the process of printing a pixel line in 2 or more time frames will increase the length of the total time for printing a line.
  • the transport of the graphical medium is normally of such a kind that medium transport will occur outside the time frame when the actual pixel printing happens. But this is only theory.
  • the real movement of the graphical medium is rather complex because of the many mass-spring systems present in the system. For example mostly a rubber roller is used for pressing the medium against the nib line of the printer. This is a very elastic medium with distributed mass.
  • the friction forces between the medium and the print head mostly also depend strongly on the thermal state of the nib line as the emulsion layer will undergo some hardness variations when heated up, this with the purpose of increasing diffusion processes inside the material for accelerating the image forming process.
  • the drive system consisting of an electrical motor (reluctance based, PM based or mixed), belt systems, gears, . . . etc. also adds equivalent springs and inertia to the drive system. Because of the rapid acceleration and deceleration wanted regarding the medium transport, vibrations will be present on the transient phase of the movement. This means that when printing one group of pixels on the image forming material, it is not always guaranteed that the medium will be in exactly the same position when printing the next group of pixels. The more time is present between the printing of these 2 (or possibly even more) groups of pixels, the more chance one might have that vibrations on the medium transport will give a misalignment of the graphical output of these pixel groups. This will lead to Moiré effects in the graphical output and is not allowed.
  • Adjacent nibs are mostly thermally linked with each other. Heat transport from one nib to another occurs, mostly by conductive means, partly by radiative means. E.g. with reference to FIG. 1 , when printing the A-pixels, a lot of heat will be transferred to the B-nibs, giving in practice a substantially increased graphical output depending on the thermal coupling between the A and B-nibs. Again, different pixel size between the several printed pixel groups may be found, giving again Moiré effects in the graphical output.
  • the electrical resistance is formed by the deposition of a continuous track of a resistive conductive paste on a substrate, as shown in FIG. 2 , e.g. using a screening technique.
  • Electric contact fingers can already be present on this substrate or can be deposited later on the surface of the resistive nib line itself.
  • the nib track forms a continuous thermal structure without any barriers for heat inside.
  • the individual nibs are formed by a delimitation of the electrical current configuration due to the location of the electrical contact fingers. But for heat, there is no delimitation, making that heat will always spread along the nib line when generated in one of the individual ‘nibs’.
  • a control algorithm must determine for every nib of the thermographic print head the amount of energy that must be dissipated in the resistive element. Depending on the thermal construction of the thermal head, this can be a very simple controller, e.g. all nibs are isolated from each other, giving no visual interaction on the printed medium between the several pixels. But in practice, the controller algorithm must deal with a variety of real-world problems.
  • thermographic medium giving different pixel sizes for a same nib energy, e.g. some examples:
  • a third problem is that the thermal process itself produces an excessive amount of heat which is not absorbed by the image forming medium. This excessive heat is absorbed by a heat sink, but nevertheless, gives rise to temperature gradients internally in the head, giving offset temperatures in the nibs and between the plurality of nibs. E.g. when the image forming process must have an accuracy of 1° C. in the image forming medium, an increased offset temperature of 5° C. in the heat generating element must be taken into account when calculating the power to be applied to that element.
  • a fourth problem is that the heat generating elements are in the ideal case fully thermally isolated from each other. In practice however, this is never the case and cross-talk between the plurality of nibs occurs. This cross-talk can be localised on several levels:
  • a further problem is that the electrical excitation of the nibs does mostly not happen on an isolated base. This means that not every nib resistor has its own electrical voltage supply which can be driven independent of all the other nibs. In general, some drive signals for driving the nibs are common to each other, this with the purpose of having reduced wiring and drive signals. In general, all nibs can be only switched on or off in the same time-frame. Producing different weighted excitations can only be achieved by dividing the excitation interval in several smaller intervals, where for every interval it can be decided whether the individual nib has to be switched on or off. This process of “slicing” has its influence on the thermal image forming process.
  • giving a pattern excitation with the weights (or driving times) (128,0,0,0,0,0,0,0) and (0,64,32,16,8,4,2,1) is mathematically only 1 point different, but the pixel size will be much more different than just 1 point in case of a commercial thermal head, because a ‘0’-no excitation interval produced in that specific device, produces heat in the nib as well!
  • the controller has to take this effect into account.
  • sub line time One way to counter-act on cross-talk is by making the active print period of each sub line, also called sub line time hereinafter, as short as possible.
  • a minimal time is present for each sub line, as the heater elements have a limit on the thermal power they can deliver and a minimum input power is necessary for the thermographic material to produce an image forming chemical reaction.
  • the disadvantage of using a short sub line time is the fact that the controllability of the whole system is minimised, as there is no or little time left to produce numerous time slices, a technique necessary to control the power to the plurality of heater elements when being driven all by a common strobe signal (e.g.
  • the above objectives is accomplished by a method and device according to the present invention.
  • the print quality is increased, while retaining the number of sub lines to a minimum and allowing for larger sub line times and accordingly more time slices and increased controllability. Therefore, an improved control strategy when printing the sub lines is provided.
  • the present invention provides a method for reducing cross-talk between pixel areas printed in a line on a thermographic material by a thermal printing system comprising a thermal printer with a thermal head having a set of energisable heater elements.
  • the energisable heater elements are drivable with at least one activation pulse for supplying a controllable amount of heat to the heater elements to generate a graphical output level of pixel areas on the thermographic material.
  • the method is characterised by sequentially driving a plurality of subsets of the heater elements to print pixel areas in each line, and reducing the cross-talk between pixel areas printed by heater elements in the same and/or different subsets by calculating a value relating to heat supplied to an n th heater element in accordance with a predetermined relationship relating the effect of heat from any one heater element after activation thereof on the graphical output of neighbouring heater elements in the same and/or a different subset, and driving the n th heater element in accordance with the calculated value.
  • the predetermined relationship may be a discrete set of coefficients relating the effects of heat from one heater element after activation thereof on the graphical output of neighbouring heater elements in space and time.
  • the predetermined relationship is in the form of a matrix.
  • This matrix has coefficients, which may be found on an experimental a posteriori base by using a special graphical printout of pixels chosen in such a way that a graphical output level is influenced by a single neighbouring pixel with a corresponding heat transfer coefficient, allowing to adjust this coefficient until the graphical output level is identical to the same graphical output level when being printed when p is not excited.
  • the number of subsets of the heater elements may be at least two.
  • a method according to the present invention may furthermore comprise line to line latent heat compensation.
  • a method according to the present invention may comprise the steps of: building system equations that relate the excitation an actual heater element will get as a result of the contributions of the neighbouring heater elements being driven, based upon the predetermined relationship, the actual heater element excitation and the non-image related sub line heat production vector, for every line to be printed, putting the total excitation value equal to a first reference value for every pixel that will be printed and equal to a second value for every pixel not being printed,
  • the second value may be calculated from the system equations using for the first time the first reference value for the excited heater elements and in subsequent iterations, the excitation values found at the heater elements being excited and a zero-value at the non-excited heater elements.
  • heater element n focus only on the equivalent steering time t r,n total in the sub line r, the actual sub line wherein the heater element is actively excited, giving in total N nibs equations for N nibs unknown excitation values.
  • the basic convolutional expression may be replaced by an expression giving an isolated boundary condition in the thermal head:
  • the present invention also provides a control unit for use with a thermal printer for printing an image onto a thermographic material, the thermal printer having a thermal head having a set of energisable heater elements, the control unit being adapted to control the driving of the heater elements with at least one activation pulse for supplying a controllable amount of heat to the heater elements to generate a graphical output level of pixel areas on the thermographic material, the control unit furthermore being adapted for controlling the driving of a plurality of subsets of the heater elements to print pixel areas in each line, and for reducing the cross-talk between pixel areas printed by heater elements in the same or different subsets by calculating a value relating to heat supplied to a first heater element in accordance with a predetermined relationship relating the effect of heat from one heater element after activation thereof on the graphical output of neighbouring heater elements in the same and/or different subsets, and for driving the first heater element in accordance with the calculated value.
  • the present invention furthermore provides a thermal print head provided with a control unit according to the present invention.
  • the thermal print head may be a thin film head; According to another embodiment, the thermal print head may be a thick film head.
  • the present invention also provides a computer program product for executing any of the methods of the present invention when executed on a computing device associated with a thermal print head, and a machine readable data storage device storing the computer program product of the present invention.
  • FIG. 1 shows an example of a thick film nib line structure having electrical contact fingers to the nib line at 300 dpi but allowing to print at 600 dpi by sharing two nibs to a same electronics switch and with additional switching on the Va and Vb voltages with which the present invention can be used.
  • FIG. 2 is a perspective view of a thick film thermal print head showing the nib track deposited on a substrate with which the present invention can be used. The electrical contact fingers are not shown.
  • FIG. 3 is a printout with each line 1 pixel (d micrometers) wide, the lines being printed with a periodicity ⁇ .
  • FIG. 4 is a schematic overview of a driver structure of a thermal head consisting of a controller and a slicer which realises the requested nib driving times with which the present invention can be used.
  • FIG. 5 shows some basic functions of a direct thermal printer with which the present invention can be used.
  • FIG. 6 shows a control circuitry in a thermal print head comprising resistive heater elements with which the present invention can be used.
  • FIG. 7 illustrates the influence of the heat transfer coefficient H i,j (i is sub line number, j relative neighbour number) by printing 2 distinct lines, a first line with pixels at nib n and n+j and a second line with only a pixel at nib n+j.
  • Correct tuning of H i,j in the deconvolution algorithm according to the present invention should make the pixel at line 1 equal size or equal dense as in line 2 , which serves in this case as a reference.
  • An “original” is any hardcopy or softcopy containing information as an image in the form of variations in optical density, transmission, or opacity.
  • Each original is composed of a number of picture elements, so-called “pixels”. Further, in the present application, the terms pixel and pixel area are regarded as equivalent.
  • the term pixel may relate to an input image (known as original) as well as to an output image (in softcopy or in hardcopy, e.g. known as a print or printout).
  • thermographic material (being a thermographic recording material) comprises both a thermosensitive imaging material and a photothermographic imaging material (being a photosensitive thermally developable photographic material).
  • thermographic imaging element is a part of a thermographic material.
  • thermographic imaging element comprises both a (direct or indirect) thermal imaging element and a photothermographic imaging element.
  • thermographic imaging element will mostly be shortened to the term imaging element.
  • heating material is meant a layer of material which is electrically conductive so that heat is generated when it is activated by an electrical power supply.
  • a heater element is a part of the heating material.
  • a “heater element” (also indicated as “nib”) being a part of the heating material is conventionally a rectangular or square portion defined by the geometry of suitable electrodes.
  • a “platen” comprises any means for firmly pushing a thermographic material against a heating material, e.g. a drum or a roller.
  • a heater element is also part of a “thermal printing system”, which system further comprises a power supply, a data capture unit, a processor, a switching matrix, leads, etc.
  • a “heat diffusion process” is a process of transfer of thermal energy (by diffusion) in solid materials.
  • activation pulse is an energy pulse supplied to a heater element, described by a certain energy given during a defined time interval ts.
  • the elementary time interval ts during which a strobe signal is active is often called a “time slice”.
  • time slice of activation pulses explicitly indicates that during a time slice, and hence during a same strobe signal, the individual heater elements may be individually and independently activated or non activated by corresponding activation pulses.
  • control of a thermal printing system denotes the ability to precisely control the output of a pixel, independent from the position of the pixel, the presence of pixel neighbours, the environmental conditions and the past thermal history of the printing process.
  • composition denotes the process of determining the exact amount of thermal energy that has to be delivered to a heater element in order to achieve a controlled graphical output.
  • a “specific mass ⁇ ” is a physical property of a material and means mass per volumetric unit [kg/m 3 ].
  • a “specific heat c” means a coefficient c describing a thermal energy per unit of mass and per unit of temperature in a solid material at a temperature T [J/kg ⁇ K].
  • thermo conductivity ⁇ is a coefficient describing the ability of a solid material to conduct heat, as defined by Fourier's law
  • the process of printing a single pixel line in several time frames, each time addressing different or even the same subset of heater elements of a thermographic print head, will be denoted in the present patent application as a printout using several sub lines.
  • the first sub line might consist of printing pixel areas using only the A-nibs
  • the second sub line might consist of printing pixel areas using only the B-nibs.
  • more exotic printing schemes could also be used, e.g. in every sub line, every fourth nib prints a pixel area, if necessary (depending on the content of the image to be printed): in sub line 1 , nibs A 4 , A 8 , . . .
  • nibs A 1 , A 5 , A 9 , . . . can be driven, in sub line 3 , nibs B 2 , B 6 , B 10 , . . . can be driven and finally in sub line 4 the nibs B 3 , B 7 , B 11 , . . . can be driven.
  • all kind of configurations can be considered when composing sub lines, but in the end all the pixel areas on that line will have been printed.
  • One reason for using sub lines is based on the limitation of the control electronics. There can, however, be other reasons, not based on limitations of the electrical system. For example one can introduce some waiting time between the sub lines with the purpose of having a small cooling period. This diminishes the cross-talk effect between the heater elements having printed in the past, and the heater elements that will be printing in the near future. Because of parasitic heat coming from one nib and flowing to the others, a small waiting period can give a sufficient reduction to the nib temperature producing in that case no fog on the image forming material. Also, when compensation is not possible, a short waiting period can make an uncompensated pixel acceptable.
  • every time slice representing a quantified amount of energy that is being delivered to the heater element (e.g. explained in U.S. Pat. No. 5,786,837).
  • the more time slices the more resolution is available to drive every heater element. In practice, this will enlarge the total time necessary for printing a sub line and this increase in time will increase the cross-talk between the active nibs, despite the increased controllability of every heater element energy. This increased cross-talk effect will be found in more pronounced Moiré effects on the graphical output.
  • a picture is considered that is being printed and which consists of simple vertical lines, as represented in FIG. 3 .
  • Each line is one pixel or d micrometers wide, and the lines are printed with a periodicity ⁇ .
  • the density measured will theoretically be given by:
  • a print process is considered where N s sub lines are being used for printing a single line.
  • the time between every sub line is t ss and is assumed now, as an example only, to be a constant, although the theory can easily be extended for non constant inter sub line times, making it of course more complex.
  • the thermal system can be considered as being a linear system, this is that the thermal properties ( ⁇ , ⁇ overscore ( ⁇ ) ⁇ ,c) of the system will remain constant (this is not a function of time).
  • the thermal system is then fully described by
  • the superposition principle applies for the thermal system in the printer and will be correct for the temperature distribution in the image forming material, but it does not apply to the graphical output, because the image forming process itself is nonlinear, excluding every use of linear superposition and convolution.
  • the aim of compensation is to be able to reproduce the same pixel under all circumstances. That means that for different circumstances, one will try to reproduce a temperature image in the graphical material, that is the same under all circumstances, e.g. have a pixel A and a pixel B, one aside the other.
  • the heat of sub line 1 generated for printing pixel A can be superimposed on the heat produced in the second sub line for printing pixel B.
  • the compensation algorithm is correct, pixel B will receive a smaller amount of heat, to compensate for the heat already present from printing pixel A.
  • the image forming material will see the same amount of heat coming from nib B, regardless of whether nib A was on or off. In that case, the same graphical output is obtained, although the graphical process itself is non linear. In fact, when the input of a non linear system is under all circumstances the same, the output also will be the same.
  • the amount of thermal energy in the image forming material can be expressed by an equivalent excitation time t e [ ⁇ s].
  • T ref cold nib
  • T ref reference temperature
  • the nib itself will be excited during a time t exc , being numerically different from t e .
  • the relation between t e and t exc is schematically shown in FIG. 4 . But from the viewpoint of the controller, the exact value of t exc is not important. It is the slicer's duty to realize a virtual t e value so that it looks for the controller as if it were working with a linear printing process. Details concerning a slicer construction can be found in EP-1234677.
  • the above expression is in the temperature domain.
  • a relationship between the temperature domain and the t e domain it is necessary for every nib to calculate a representative temperature value in the thermal sensitive material under the nib when being excited with a t e value.
  • This is e.g. a mean temperature value or a complicated function taking into account the thermographic characteristics of the image forming medium.
  • the maximum mean temperature value will be used, only for the sake of explaining this matter.
  • T pixel ⁇ ( t e ) max ⁇ [ ⁇ pixel ⁇ ⁇ T ⁇ ( r ⁇ , t ) ⁇ d V ] . Eq . ⁇ ( 5 ) So, for every t e value, it is possible to find for that nib a representative thermal state T pixel that has a direct relationship with the graphical output.
  • Table 1 can theoretically be done using numerical techniques. For a certain excitation time t e , the temperature distribution can be calculated in the thermal head, including the image forming material. Only one nib must be excited with this value and during the simulation the correct slicer pattern must be used. The simulation must comprise all sub lines and the correct timing between the different sub lines must be used, even when they are not equally spaced in time. For the considered generated pixel, the value of the representative thermal state T pixel can be calculated for all pixels and for all sub lines. By dividing all values by T ref (t e ) of the pixel found at the very first calculated sub line, one gets all values relative to the pixel written.
  • the size of the pixel response is normally limited: as the heat tends to spread in a range of several milliseconds, mostly only the direct neighbours will be affected by cross-talk. So in the horizontal sense, the pixel response will be limited. In the sub line direction, the limitation comes most often from the number of sub lines itself, as too many sub lines is difficult to combine with a fast transport rate of the thermographic medium and as it normally gives too large line times, being economically unacceptable.
  • the printing process using several sub lines can be regarded as a process of creating latent heat in every sub line that has to be coped with in the following sub lines to be printed. It is numerically not difficult to calculate the latent heat that will be present at the start of a sub line. Whenever the pixel transfer function is known (all of its coefficients), by making simple multiplications and additions, the latent heat in every sub line, generated by the older sub lines in the same line, can be calculated.
  • the invention here described gives a method to do compensation when printing the several sub lines in a line having cross-talk between pixels being printed in the same sub line.
  • sub line printing could be interpreted as sequentially printing several lines without medium movement, this is not fully true. Adjacent pixels will interact with each other because the heat transport from one to the other is so fast that they will influence each other. This is certainly the case when the sub line time is taken large in order to improve the controllability of the printing process using more time slices.
  • N nibs is the total number of pixels on the line. Whenever no pixel is printed, its value will be set to zero, in the other case its value will be a constant t ref or t ref corrected with some correction factor. As the slicer will extend the print job over several sub lines r, for every sub line it is necessary to give a more precise definition of what pixel temperature is desired.
  • the slicer will distribute this line information over the several sub lines r.
  • H r,k be the pixel response function, with the r-index the number of the sub line and k the neighbour nib number. H 0,0 will be equal to 1.
  • the H-matrix is symmetrical, this means that nibs at position x+k will see the same heat as the nibs at position x ⁇ k.
  • t r add is an additional term and represents the heat produced in nib n due to the zero-excitation energy from all the other nibs and integrating as well the effect (0-excitation energy) of the former sub lines.
  • Some thermal head constructions have the property that heater elements not being addressed during an active strobe time, still deliver some fixed amount of energy (e.g. U.S. Pat. No. 5,702,188). It is only assumed that this parasitic off-switched heat generation during the printing process is the same for all the nibs. In that case, this heat generation can be bundled into a single constant, being different for each sub line. For the first sub line, t 0 add can be taken equal to zero. This is just a matter of references.
  • the coefficients of H can be neglected when the i-index becomes large, i.e. for nibs far away from the excited nib.
  • i-index i.e. for nibs far away from the excited nib.
  • all the H-coefficients where zero.
  • Errors happen only at the outer ends of the printable region, so they are in most cases not visible.
  • some more complicated boundary conditions can exist at the end of a thermal head, making the assumption of a thermal symmetry plane not very credible.
  • a more correct modelling can be looked for, but again, because of the limited H-span, only small errors will be present at the thermal head boundary, so that again Eq.(6) will do the job.
  • relaxation being also an embodiment of the invention.
  • Relaxation is in fact built on an iteration process.
  • the excitation of these nibs must be known, something which is not true a priori.
  • Relaxation is then built on supposing an a priori solution, calculating cross-talk and then finding the t r,n wanted value for the non printed pixels which are being printed in the considered sub line.
  • the system of equations can be solved, giving new values of t r,n e which can be re-used for a new cross-talk calculation, etc. . . . until a result is found which is accurate enough. This will be explained with an example.
  • t 1 relax h 1 t ref +h 1 t ref +h 2 t ref Eq.(16)
  • the controller of the printing device can fully be developed taking into account that the value of these coefficients should be user selectable (at run-time or at compile time, requesting of course successive recompilation). Although being unknown, they can always be taken 0, giving in fact an uncompensated printing device.
  • the t r add coefficients are determined.
  • the t r add has to make a pixel printout in the sub line r identical to a pixel printed in the other sub lines, given that the pixel is printed without any neighbours (or in fact excluding the effect from the other cross-talk coefficients).
  • the constant t 0 add can be taken zero, meaning in fact that a pixel with index 4 i is the reference in our printing scheme.
  • the 4 i pixel is printed in sub line 0
  • the same pixel is printed in another sub line r ⁇ 0.
  • the pixel By adjusting the corresponding t r add value, the pixel should be made equal sized or equal dense to the reference pixel when printed in sub line 0 . Comparing the same pixel with itself has the benefit that there is no interference with mechanical print differences between several nibs, being present because of constructional fabrication differences.
  • the cross-talk coefficients (Table 2) are determined. As the pixel data is distributed over the several sub lines when printing a single line, only those coefficients must be considered in a sub line where actual pixel data is being printed. So H i,j is important when in sub line i, the j-th or -j-th neighbour is printed.
  • At least one printing pattern can be defined where the coefficient H i,j will be the only coefficient active in the printing process. Again, a pixel has to be compared with itself and the value of the coefficient H i,j is adapted until the pixel becomes equal sized or equal dense. When making the printouts, the values of the other coefficients don't need to be taken 0, meaning that for these cross-talks effects, the compensation can be active, although it will have no influence on the current printing process.
  • a print pattern for observing the effect of the coefficient H i,j as is depicted in FIG. 7 This can be illustrated with a print pattern for observing the effect of the coefficient H i,j as is depicted in FIG. 7 .
  • two distinct lines are printed, each in a number of sub lines.
  • a first line, line 1 has printed pixels, represented by the black dots, at nib n and at nib n+j.
  • a second line, line 2 has only a printed pixel, represented by a black dot, at nib n+j.
  • Nib n is not excited, which is represented by a white dot.
  • Correct tuning of H i,j in the deconvolution algorithm according to the present invention should make the pixel generated by nib n+j at line 1 equal size or equal dense as the pixel generated by nib n+j in line 2 , which serves in this case as a reference.
  • a thermal head has a plurality of nibs with nib numbers ⁇ 0, 1, 2, 3, 4, . . . , i, i+1, i+2, i+3, i+4, . . . , N nibs ⁇ 1 ⁇ .
  • One line is printed in two sub lines. In sub line 0 , all pixels with index or nib numbers 4 i and 4 i+ 2 are being printed; in sub line 1 , all pixels with index 4 i+ 1 and 4 i+ 3.
  • t 4i+1 nib t 4i+1 e + ⁇ 1 t 4i e + ⁇ 1 t 4i+2 e + ⁇ 2 t 4i ⁇ 1 e + ⁇ 2 t 4i+3 e +t 1 add .
  • t 4i+3 nib t 4i+3 e + ⁇ 1 t 4i+2 e + ⁇ 1 t 4i+4 e + ⁇ 2 t 4i+1 e + ⁇ 2 t 4i+5 e +t 1 add .
  • variable image data is present, an iterative solution process is followed, this with the purpose of finding a best physical solution which can be applied during the printing process.
  • the vector t n e is initialised according the image information:
  • a vector t n relax is resolved using:
  • a print pattern can be used for isolating the effect of every coefficient.
  • each coefficient can be tuned until all pixels are equal-sized or equal-dense for the given pattern.
  • the following pattern can be used.
  • Pattern ⁇ ⁇ 3 ⁇ ⁇ ⁇ ( for ⁇ ⁇ ⁇ 2 ) ⁇ [ 1 4 ⁇ i 0 1 4 ⁇ i + 2 0 0 1 4 ⁇ i + 1 0 1 4 ⁇ ( i + 1 ) + 3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 4 ⁇ ( i + 1 ) + 3 0 0 0 0 0 0 0 0 0 0 0 0 0 1 4 ⁇ i + 2 0 0 0 0 1 4 ⁇ ( i + 1 ) + 3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ] , which shows two print lines, giving the interaction between the 4 i and the 4 i+ 2 pixel and also between the 4 i+ 1 and the 4 i+ 3 pixel.
  • the coefficient ⁇ 2 must be chosen in such a way that the 4 i+ 2 pixel printed adjacent to the 4 i pixel is equal sized or equal dense to the 4 i+ 2 pixel printed isolated (first line in Pattern 3). In fact two different values can be found for ⁇ 2 as there are in this case two different experiments possible ( 4 i+ 2 influenced by 4 i and 4 i+ 1 influenced by 4 i+ 3). When the cross-talk model would be correct, all the values of ⁇ 2 found would be the same. When different values of ⁇ 2 are found, an error probably is present in the cross-talk model (Eq.(21)), meaning that coefficients taken zero in the cross-talk matrix in fact are not zero. In that case, cross-talk coefficients must be added and the whole compensation algorithm has to be redone.
  • thermographic recording material m comprises on a support a thermosensitive layer, and generally is in the form of a sheet.
  • the imaging element 5 is mounted on a rotatable platen or drum 6 , driven by a drive mechanism (not shown) which continuously advances (see arrow Y representing a so-called slow-scan direction) the drum 6 and the imaging element 5 past a stationary thermal print head 20 .
  • This head 20 presses the imaging element 5 against the drum 6 and receives the output of the driver circuits (not shown in FIG. 1 for the sake of greater clarity).
  • the thermal print head 20 normally includes a plurality of heater elements equal in number to the number of pixels in the image data present in a line memory.
  • the image wise heating of the heater element is performed on a line by line basis (along a so-called fast-scan direction X which generally is perpendicular to the slow-scan direction Y), the “line” may be horizontal or vertical depending on the configuration of the printer, with the heater resistors geometrically juxtaposed each along another and with gradual construction of the output density.
  • Each of these resistors is capable of being energised by heating pulses, the energy of which is controlled in accordance with the required density of the corresponding picture element.
  • the output energy increases and so the optical density of the hardcopy image 7 on the imaging element 5 .
  • lower density image data cause the heating energy to be decreased, giving a lighter picture 7 .
  • the activation of the heater elements is preferably executed pulse wise and preferably by digital electronics. Some steps up to activation of said heater elements are illustrated in FIG. 5 and FIG. 6 .
  • input image data 16 are applied to a processing unit 18 .
  • a stream of serial data of bits is shifted (via serial input line 21 ) into a shift register 25 , thus representing the next line of data that is to be printed.
  • these bits are supplied in parallel to the associated inputs of a latch register 26 .
  • a strobe signal 24 controls AND-gates 27 and feeds the data from latching register 26 to drivers 28 , which are connected to heater elements 29 .
  • These drivers 28 e.g. transistors
  • the recording head 20 is controlled so as to produce in each pixel the density value corresponding with the processed digital image signal value. In this way a thermal hard-copy 7 of the electrical image data is recorded. By varying the heat applied by each heater element to the carrier, a variable density image pixel is formed.
  • the thermal printing apparatus 10 is therefore provided with a control unit 30 .
  • the control unit 30 may include a computing device, e.g. microprocessor, for instance it may be a microcontroller.
  • a programmable printer controller for instance a programmable digital logic element such as a Programmable Array Logic (PAL), a Programmable Logic Array, a Programmable Gate Array, especially a Field Programmable Gate Array (FPGA).
  • PAL Programmable Array Logic
  • FPGA Field Programmable Gate Array
  • This control unit 30 is adapted to drive the heater elements in subsets to print pixel areas in each line so as to form sub lines.
  • the control unit 30 is furthermore adapted for reducing the cross-talk between pixel areas printed by heater elements in the same or different subsets by calculating a value relating to heat supplied to a first heater element in accordance with a predetermined relationship relating the effect of heat from one heater element after activation thereof on the graphical output of neighbouring heater elements, and for driving the first heater element in accordance with the calculated value.
  • the heater elements may be electrically excited heater elements based on the Joule effect, directly (conductively) or indirectly (capacitively, inductively or RF) supplied from a voltage source.
  • the heater elements may be based on a light or IR to heat conversion.
  • the heater elements may be based on exothermal chemical, biological or pyrotechnic controllable reactions. Applications can be found in the field of half-tone printing, using equal sized and equal dense pixels or the continuous tone printing, having pixels with varying density. The present invention can be applied both in greyscale or binary printing and for printing colour images with photographic quality.
US10/738,816 2002-12-17 2003-12-17 Deconvolution scheme for reducing cross-talk during an in the line printing sequence Expired - Fee Related US7023461B2 (en)

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EP1431044A1 (en) 2002-12-17 2004-06-23 Agfa-Gevaert A deconvolution scheme for reducing cross-talk during an in the line printing sequence
DE102005040413A1 (de) * 2005-08-26 2007-03-15 Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. Vorrichtung zum Aufzeichnen von Daten auf einer lichtempfindlichen Datenträgeranordnung

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EP1431044A1 (en) 2002-12-17 2004-06-23 Agfa-Gevaert A deconvolution scheme for reducing cross-talk during an in the line printing sequence

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US4360818A (en) 1979-11-28 1982-11-23 Fuji Xerox Co., Ltd. Heat-sensitive recording head with minimum number of switching diodes
US4366489A (en) * 1980-02-06 1982-12-28 Ricoh Company, Ltd. Thermal recording apparatus
EP0304916A1 (en) 1987-08-28 1989-03-01 Nec Corporation Thermal printing control circuit
US5719615A (en) 1989-03-09 1998-02-17 Kyocera Corporation Apparatus for driving heating elements of a thermal head
US5483273A (en) 1991-02-26 1996-01-09 Rohm Co., Ltd. Drive control apparatus for thermal head
US5815191A (en) * 1995-01-31 1998-09-29 Agfa-Gevaert Direct thermal printing method and apparatus
US6008831A (en) 1995-02-23 1999-12-28 Rohm Co., Ltd. Apparatus for controlling driving of thermal printhead
US5702188A (en) 1995-07-18 1997-12-30 Graphtec Corporation Thermal head and head drive circuit therefor
FR2808476A1 (fr) 2000-05-04 2001-11-09 Sagem Procede de commande d'une tete ligne d'impression thermique
EP1234677A1 (en) 2001-01-25 2002-08-28 Agfa-Gevaert Method for thermal printing
EP1431044A1 (en) 2002-12-17 2004-06-23 Agfa-Gevaert A deconvolution scheme for reducing cross-talk during an in the line printing sequence

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